In optical transmitters that utilize semiconductor lasers as the optical source, the laser may be either directly modulated or externally modulated. In the directly modulated case the drive current to the laser is modulated in accordance with an information signal in order to produce a corresponding modulation of a parameter (e.g., intensity) of the output beam of the laser. In the externally modulated case the laser is operated in a continuous wave (cw) mode, and the output beam of the laser is coupled to an optical modulator that is external to the laser. An information signal is applied to the modulator so as to modulate a parameter of the output beam.
In a typical externally modulated optical transmitter the modulator is a semiconductor electroabsorption (EA) modulator. The EA modulator relies on the Quantum-Confined Stark Effect (in MQW semiconductors) or the Franz-Keldysh Effect (in bulk semiconductors) to alter the absorption of the laser beam. That is, a voltage bias applied to the EA modulator causes the bandgap of the modulator to shift relative to the wavelength of the beam which, in turn, changes the absorption of the beam.
The difference between the wavelength of the laser beam and the wavelength corresponding to the bandgap of the EA modulator is defined as the detuning. Detuning controls many important transmission parameters such as output power, extinction ratio and dynamic chirp of the modulator.
In a single wavelength transmitter, such as a monolithically integrated DFB laser/EA modulator, detuning is tightly controlled by the design of the devices; e.g., by varying the bandgap of the EA modulator during epitaxial growth to match the desired DFB wavelength on a particular wafer. One or two nanometers of variability can be compensated by adjusting the voltage bias to the modulator. The adjustment range is limited, however, by the voltage at which the EA modulator characteristics become degraded.
In WDM systems the transmitter is modified so that it is capable of generating an output beam at a any one of a multiplicity of wavelengths. One such modification is to replace the single-channel DFB laser with either a broadband wavelength selectable laser (WSL), such as a tunable DBR laser, or an array of DFB lasers coupled to a passive combiner network. In this case, however, the detuning of each channel wavelength from the EA modulator bandgap would be different from channel to channel, with the undesirable consequence that the transmission performance of each channel would be different.